In pharmacology, Enteral (Greek, enteros , meaning :intestine") is any route of drug administration that involves absorption of the drug through the gastrointestinal tract. This contrasts with the parenteral (Greek para: "besides" + enteros), meaning outside the intestines, or more specifically in pharmacology: Methods of introducing drugs or substances into the body other than through the gastrointestinal tract, via, for example, intravenous injection.
Forms of enteral administration 
Enteral administration may be divided into three different categories, oral, gastric, and rectal. (Gastric introduction involves the use of a tube through the nasal passage or a tube in the belly leading directly to the stomach.)
Drug absorption from the intestine 
The mechanism for drug absorption from the intestine is for most drugs passive transfer, a few exceptions include levodopa and fluorouracil, which are both absorbed through carrier-mediated transport. For passive transfer to occur, the drug has to diffuse through the lipid cell membrane of the epithelial cells lining the inside of the intestines. The rate at which this happens is largely determined by two factors: Ionization and lipid solubility.
Factors influencing gastrointestinal absorption:
First pass metabolism 
Drugs given by enteral administration may be subjected to significant first pass metabolism, and therefore, the amount of drug entering the systemic circulation following administration may vary significantly for different individuals and drugs. Rectal administration is not subject to extensive first pass metabolism.
See also